Well, I can't say for certain that it is true but from experience I know that the older the egg, the higher up in a glass of water it will be. So while a floating egg might not be bad, it certainly is old.

I follow this advise in any case. When the egg touches the bottom of the glass, either lying down (freshest) or standing on the tip I feel comfortable using it raw like in tiramisu. If it floats in the lower half of the glass I still eat it but cooked. If it floats in the upper half I throw it out. For what it's worth, I have never eaten (or even cracked open) a bad egg.

Well, I can't say for certain that it is true but from experience I know that the older the egg, the higher up in a glass of water it will be. So while a floating egg might not be bad, it certainly is old.

That is correct. When they are fresh there isn't much air at all in there and as they age more air gets in. I don't remember or know exactly why and I don't know why it doesn't go back out, but that's what happens.

They can still be perfectly safe, though, if they were stored at the apropriate temperature.

gsm's guidelines are about what mine are, too. If it stands up a bit that's okay but one that's truly floating is quite old and eggs being so cheap, not worth the risk really. Though I think if it were rotten you'd probably know it the minute it cracked. EEEYEW.

Eggs tend to dry up in their shell over time, but normally don't turn bad.
I've eaten on several instances eggs that were about 3 months old and kept at room emperature, and lived to tell the tale. The yolk does break up more easily the longer you wait, though, all the more if the drying caused it to stick to the shell. They do acquire an odd taste, though, even when cooked up.
Though recipes involving only the white (or yolk) require fresh eggs for the two to stay separated, I think the whole dated-eggs (I mean eggs on which the day and hour is printed) snobbism is little more than marketing.
Eggs do turn bad, and a contaminated egg will eventually cause severe illness, but there is no need to be paranoid about it.
I've been taught eggs stay edible for as long as their shell remains whole and protects them from contamination. It needs to be verified, but basically I never died of food poisoning.

I have first hand experience of bad eggs floating .... and of them sinking. It depends on what putrefaction processes are going on inside the egg and what gases are being released. The shell is permeable to gases (at least to some gases), so putrefaction gases won't always stay inside the shell.

Come from a old egg farming family and learned this at a very early age. I have also read about this somewhere at much later date. It has to do with the amount of gas trapped in the egg. Fresh eggs lay on the bottom and bad eggs lay on the top (float on their sides). If on end of the egg is sticking out of the water it is still good to eat, but need to be done soon. If it starts to lay over or is lay on it side when it floats, then do not eat.